Grade11 - Biology - Cell - Division - Cell - Cycle - Metaphase

CELL CYCLE

The cell cycle, or cell-division cycle, is the series of events in a eukaryotic cell between one cell division and the next. Thus, it is the process by which a single-cell fertilized egg develops into a mature organism and the process by which hair, skin, blood cells, and some internal organs are renewed.

Phases of Cell Cycle

The cell cycle of a typical eukaryotic cell has four phases. The relatively brief M phase consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis). After M phase, the daughter cells each begin interphase of a new cycle. Although the various stages of interphase are not usually morphologically distinguishable, each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell division. The first phase within interphase is called G1 (G indicating gap); during this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated. The cell then enters the G2 phase, which lasts until the cell enters the next round of mitosis. Metabolic activity, cell growth, and cell differentiation all occur during interphase.
The term "post-mitotic" is sometimes used to refer to both quiescent and senescent cells. Non-proliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons).
The molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a sequential fashion and it is impossible to "reverse" the cycle. There are two key classes of regulatory molecules that determine a cell's progress through the cell cycle: cyclins and cyclin-dependent kinases.

Check Points

The cell to monitor and regulate the progress of the cell cycle uses checkpoints. If a cell fails to meet the requirements of a phase it will not be allowed to proceed to the next phase until the requirements have been met. Several checkpoints are designed to ensure that damaged or incomplete DNA is not passed on to daughter cells. At the end of the G1 phase, G2 phase and after DNA has been replicated in the S phase it is checked for damages. At the end of the M phase a checkpoint is present to stop cytokinesis in case the chromosomes are not properly aligned on the mitotic spindle.

MITOSIS IN PLANTS

Interphase (G1, S, G2)

Nuclear material is surrounded by a nuclear envelope. Dark-staining bodies, nucleoli, are visible. Chromosomes appear only as dark granules within the nucleus. Collectively, the chromosome mass is called chromatin. The chromosomes are not individually distinguishable because they are uncoiled into long, thin strands. Chromosomes are replicated during this phase.

Prophase

Chromosomes begin to coil and become distinguishable thin, threadlike structures, widely dispersed in the nucleus during prophase. Although there are no centrioles in plant cells, a spindle begins to form. Nucleoli begin to disappear. The nuclear envelope is still intact.

Prometaphase

By prometaphase, the chromosomes are thick and short. Each chromosome is double-stranded, consisting of two chromatids held together by a centromere. The nuclear membrane breaks down in prometaphase. Chromosomes move toward the equator.

Metaphase

Metaphase begins when the centromeres of the chromosomes lie on the equator of the cell. The arms of the chromatids extend randomly in all directions. A spindle may be apparent. Spindle fibers are attached to centromeres and extend to the poles of the cell. As metaphase ends and anaphase begins, the centromeres split.

Anaphase

The splitting of centromeres marks the beginning of anaphase. Each former chromatid is now a chromosome. Single-stranded chromosomes are drawn apart toward opposite poles of the cell. Anaphase ends when the migrating chromosomes reach their respective poles.

Telophase and Cytokinesis

Chromosomes have now reached the poles. The nuclear envelope reforms around each compact mass of chromosomes. Nucleoli reappear. Chromosomes begin to uncoil and become indistinct. Cytokinesis is accomplished by the formation of a cell plate that begins in the center of the equatorial plane and grows outward to the cell wall.